Battery state-of-charge indicator

Abstract

A state-of-charge indicator, powered by a battery to be monitored, compares a reference voltage level developed from the battery and a voltage proportional to that of the battery in a differential-type circuit employing a pair of transistors with common emitter load and each having a collector current activated annunciator; with a first annunciator being activated under conditions of battery voltage definitive of a "good" battery, both annunciators being activated for lower battery voltage indicative of a failing trend, only the second annunciator being activated for still lower battery voltage indicative of a "bad" battery, and neither annunciator being activated for still lower battery voltage indicative of a failed condition. The minimal current drain device is not adversely effected by voltage drop in wiring runs to the monitored battery, and may be self-contained within a plug-in unit for insertion into a vehicle cigarette lighter receptacle, or dashboard mounted, with non-critical length wiring run to the vehicle battery.

I claim: 
     
       1. A state-of-charge indicator for a battery comprising: a pair of current flow activated indicator means, first terminal means and second terminal means, a pair of transistors with collector elements serially connected through respective indicator means of said pair of current flow activated indicator means to said first terminal means, a common resistive member connected to the emitter elements of said pair of transistors with each transistor connected through said common resistive member to said second terminal means, first means biasing the base-emitter junction of said first transistor as the differential between a fixed reference potential and the voltage drop across said common resistive member, and second means biasing the base-emitter junction of said second transistor as the differential between a potential defined as a predetermined function of the voltage across said terminal means and the voltage drop across said common resistive member, and means for connecting said first and second terminal means to respective battery terminals. 
     
     
       2. The indicator of claim 1 with said predetermined function being a predetermined fractional part of the voltage across said first and second terminal means. 
     
     
       3. The indicator of claim 2, with said second biasing means comprising a resistive voltage divider network connected across said first and second terminal means, with said base element of said second transistor connected to the output tap of said voltage divider network. 
     
     
       4. The indicator of claim 3, with a zener diode connected across said first and second terminal means to be reverse biased thereby, and said reference potential comprising the voltage across respective terminals of said zener diode. 
     
     
       5. The indicator of claim 3, with diode means connected across said first and second terminal means to be forward biased thereby, and said fixed reference potential comprising the forward biased voltage-drop across said diode means. 
     
     
       6. The indicator of claim 5, with said diode means comprising a plurality of like-polarized serially interconnected diode members collectively effecting said fixed reference potential. 
     
     
       7. The indicator of claim 6, with said first and second terminal means being connected to the output junction-pair of a full wave diode bridge means, and the input junction-pair of said bridge being connectable to said battery terminals. 
     
     
       8. The indicator of claim 7, with low pass filter means including a capacitor member connected across said bridge output junction-pair, with the respective terminals of said capacitor comprising said first and second terminal means. 
     
     
       9. The indicator of claim 8, with voltage limiting means connected across said first and second terminal means and operative to limit transient voltages as may appear thereacross to a predetermined maximum value in excess of the nominal output voltage level of a battery to which said indicator is connected. 
     
     
       10. The indicator of claim 8, with said current-flow activated indicating means comprising respective light emitting diodes. 
     
     
       11. The indicator of claim 1, with said respective means for biasing the base-emitter junctions of said first and second transistors effecting condition of second transistor for battery terminal defined potentials equalling and falling within a predetermined range below the fully charged output voltage of said battery, and effecting exclusive conduction of said first transistor for battery terminal means defined below a predetermined minimum level below said fully charged voltage, and effecting a lesser conduction of both said transistors for battery defined potential between said predetermined range and said predetermined minimum level. 
     
     
       12. The indicator of claim 11, with said first and second terminal means being connected to the output junction-pair of a full wave diode bridge means, and the input junction-pair of said bridge being connectable to said battery terminals. 
     
     
       13. The indicator of claim 12, with low pass filter means including a capacitor member connected across said bridge output junction-pair, with the respective terminals of said capacitor comprising said first and second terminal means. 
     
     
       14. The indicator of claim 13, with voltage limiting means connected across said first and second terminal means and operative to limit transient voltages as may appear thereacross to a predetermined maximum value in excess of the nominal output voltage level of a battery to which said indicator is connected. 
     
     
       15. The indicator of claim 13, with said current-flow activated indicating means comprising respective light emitting diodes.